1. Field of the Invention
The present invention is directed to an improved resilient seal used for isolating lubricated machine parts such as bearings, from the external environment. More particularly, the present invention is directed to an improved O-ring type seal for lubricated bearings of rock bits used for subterranean drilling.
2. Brief Description of the Prior Art
Seals for confining lubricants in machine parts, and particularly O-ring type resilient seals, are old and well known in the art.
As is known, rock bits employed for subterranean drilling when prospecting for oil, gas or minerals, are exposed to a very harsh environment, the present very difficult sealing problems. More specifically, the temperature of the subterranean formation where the rock bit is used, may reach or exceed 280 to 400 degrees F., and the ambient pressure may typically reach several thousand PSI. Moreover, the rock bit is surrounded by drilling mud which is continuously circulated from the surface. One of the functions of the drilling mud is to remove cuttings generated by the rock bit. The drilling mud, and often the subterranean formation itself, have highly corrosive chemical properties and contain highly abrasive particles.
Rock bits designed to operate under the above-mentioned conditions are described, e.g. in U.S. Pat. Nos. 4,358,384 and 4,372,624. Briefly, these and like rock bits wherein the improved seal of the present invention may be most advantageously employed, have a plurality (typicaly three) hollow cutter cones rotatably mounted on journals integral with the drill body. The drill body is rotated about its longitudinal axis either by a rotating drill string driven from the surface, or by a downhole drilling motor.
Each cutter cone has a plurality of cutting teeth. As the teeth of the cutter cones engage the subterranean formation and as the journals are rotated about the longitudinal axis of the drill body, each cutter cone rotates on a bearing relative to its respective journal. The bearing, typically a journal bearing, is lubricated by an internal supply of a suitable lubricant.
Subterranean drilling is very costly. Even the maintenance of a drilling rig and crew at the drilling site is very costly although actual drilling may not occur. Therefore, the procedure required for replacing at great well depth a defective rock bit with a functioning bit, and the accompanying interruption in drilling are also very expensive. For these reasons the useful downhole or service life of rock bits is of great importance to the drilling industry.
Whereas the service life of a rock bit depends on the integrity and proper functioning of many components of the bit, experience has shown that journal bearings of the bits fail rapidly after drilling mud or other extraneous material gains entry into the bearing.
In order to prevent ingress of drilling mud and other extraneous material to the bearings, and also to prevent loss of the lubricating grease into the environment, the prior art has utilized O-ring seals placed between the journal and its rotating cutter cone. However, it was discovered relatively early in the prior art that ordinary O-ring seals capable of providing adequate sealing in less exacting applications are inadequate in rock bits. For this reason, for specific "downhole", rock bit applications, the prior art has provided certain refinements and improvements over the basic O-ring technology.
One such improvement is disclosed in U.S. Pat. No. 3,397,928. In accordance with this disclosure, a resilient O-ring seal is mounted into an annular channel or groove formed between the journal shaft of the rock bit and the hollow cutter cone, in such a manner that, at the time of assembly, the O-ring is compressed or squeezed at least ten percent relative to its original thickness. Compression or squeeze of the O-ring improves its ability to perform under the adverse conditions of subterranean drilling. The O-ring described in the 3,397,928 patent is typically made of butadiene acrylonitrile (Buna N) rubber. In fact, this rubber material is commonly used in O-rings and like seals and packings, where the O-ring seal or packing is exposed to hydrocarbon liquids or hydrocarbon based lubricants.
U.S. Pat. No. 4,372,624 assigned to the same assignee as the present application describes a seal for a rock bit wherein an O-ring is placed into an annular chamber formed between the journal shaft and the hollow cutter cone of the rock bit. The chamber has at least one, preferably two substantially V shaped walls into which the O-ring is progressively pressed whenever fluid pressure on one side of the O-ring significantly exceeds pressure on the other side of the O-ring.
Another improvement over conventional O-ring seals specifically adpated for "downhole", rock bit applications, is described in the application, of William J. Neilson for U.S. Patent, Ser. No. 378,454, filed on May 14, 1982. The Neilson application is assigned to the same assignee as the present application. In accordance wth the teachings of the Neilson application, an annular chamber or seal gland of a rock bit bearing journal and cutter cone assembly has curved walls configured in such a manner that pressure differentials acting on the two sides of an O-ring placed into the chamber, force the O-ring to move parallel with the sealing direction into progressively greater contact with the curved walls of the seal gland.
In other words, in accordance with the Nielson application and also with U.S. Pat. No. 4,372,624 compression or squeeze of the O-ring is temporarily increased and better sealing capability is achieved whenever a relatively large pressure differential acts on the O-ring. As is known, excessive pressure differential normally presents a greater likelihood that fluid or materials may leak through the seal in either direction.
Although the above summarized O-ring seal assemblies provide acceptable sealing capabilities for downhole operation of rock bits, rock bit failures due to failed bearing seals are still common in the industry.
A principal disadvantage of prior art O-ring seals, and particulary of the prior art O-ring seal assemblies used for sealing rock bit bearings, is that the seals do not generally compensate for wear and compression set, or both which occur during use. Furthermore, highly compressed or squeezed seals of the prior art often undergo permanent deformation or "compression set" even before actual use, so that a good portion of the resiliency and "squeeze reserve" of the seal is lost.
In this regard it is noted that in the oil drilling industry approximately 120 hours of uninterrupted service life of a rock bit is considered reasonably good. Thus, the O-ring seals are ideally expected to perform without failure for approximately 120 hours, and preferably longer. The seals, however, undergo significant wear between the bearing journal and the rotating cutter cone, and seal failures in less than 120 hours of rock bit operation are still fairly common in the prior art. The industry is, therefore still striving to improve the wear resistance and hence the useful service life of rock bit seals. The present invention is directed to such an improvement.